Scroll type machines are becoming more and more popular for use as compressors in both refrigeration as well as air conditioning applications due primarily to their capability for extremely efficient operation. Generally, these machines incorporate a pair of intermeshed spiral wraps, one of which is caused to orbit relative to the other so as to define one or more moving chambers which progressively decrease in size as they travel from an outer suction port toward a center discharge port. An electric motor is provided which operates to drive or orbit the orbiting scroll member via a suitable drive shaft which is affixed to the motor rotor. In a hermetic compressor, the lower portion of the hermetic shell normally contains an oil sump for lubricating and cooling the various components of the compressor.
Scroll compressors depend upon a seal created between opposed flank surfaces of the wraps to define successive chambers for compression which generally eliminates the requirement for suction and discharge valves. However, when these compressors are shut down, either intentionally as a result of the demand being satisfied or unintentionally as a result of a power interruption or other problems, there is a strong tendency for the gas in the pressurized chambers and/or backflow of compressed gas from the discharge chamber to effect a reverse orbital movement of the orbiting scroll member and its associated drive shaft. This reverse movement often generates objectionable noise or rumble. Further, in machines employing a single phase drive motor, it is possible for the compressor to begin running in the reverse direction should a momentary power failure be experienced. This reverse operation may result in overheating of the compressor and/or other damage to the apparatus. Additionally, in some situations, such as a blocked condenser fan, it is possible for the discharge pressure to increase sufficiently to stall the drive motor and effect a reverse rotation thereof. As the orbiting scroll rotates in the reverse direction, the discharge pressure will decrease to a point where the motor again is able to overcome this pressure head and rotate the scroll member in the "forward" direction. However, the discharge pressure will again increase to a point where the cycle is repeated. Such cycling may also result in damage to the compressor and/or its associated apparatus.